In the semiconductor industry, process uniformity is extremely important, particularly as wafer sizes increase and device geometries become smaller. One approach to improving uniformity is single wafer processing, since the environment in which the wafer is processed can be more tightly controlled than in batch processing. Even in the controlled environment of single wafer processing, however, control and uniformity of wafer temperature is difficult.
In general, semiconductor wafer dry processing is conducted in process mediums at reduced pressures. This reduces the thermal conduction through the process gasses. Wafer temperature control must therefore be obtained through wafer contact conduction in some manner.
One common prior art method of mounting a wafer in a dry processing low-pressure process chamber is to simply place it face up on a flat chuck with no clamping. While this method is simple and has no clamping mechanisms to interfere with gas flow uniformity, heat conduction between the wafer and the chuck is poor. The major heat path between wafer and chuck is surface contact conduction, which is poor due to non-uniformity in contact patterns between chuck and wafer. This results in non-uniformity of temperature across the wafer. Convection and conduction through the process gas is minimal because of the low density of the gas in the process chamber.
While there is some radiation between the wafer and its environment, the temperature of the wafer depends mainly on its conduction paths to the chuck. Since it is well known that surface-to-surface thermal conduction is proportional to contact pressure, any mismatch in surface planarity between chuck and wafer will interfere with heat conduction. This also contributes to the temperature difference between the chuck and the wafer.
A number of methods of damping the wafer at its edge in order to improve wafer to chuck contact have been used, such as pins and ring clamps. Merely clamping the wafer at its circumference against a flat chuck will not produce uniform thermal contact between the wafer and the chuck. This is caused by the fact that contact pressure between the wafer and the substrate is not uniform across the wafer. Contact pressure between the wafer and the chuck will be high under the clamp, but will be lighter or zero under non-clamped portions of the wafer. The temperature of the peripherally clamped wafer is better controlled, but is still not uniform from the center to the edge of the wafer.
Another approach is to clamp the wafer against a chuck having a spherically domed surface using a full periphery clamp. This is an attempt to force the wafer against the chuck using the spring force of the wafer itself. The major disadvantage of this method is that it requires a full circumferential damp, which disturbs gas delivery uniformity and therefore contributes to non-uniformity near the edges of the wafer. In addition, the sperical dome shape is not the proper shape for a naturally deformed circular membrane in the form of a wafer. Since it is not the proper shape the wafer-chuck contact pressure is non-uniform, leading to non-uniform thermal contact, and thus to non-uniform wafer temperature.
Since surface contact conduction depends on the area of actual material to material contact, and the total area available for contact between the backside of a wafer and its chuck is on the order of 10% due to surface irregularity, even surface conduction is limited.
In order to overcome this inherent limitation in contact conduction, a helium chuck has also been used. The helium chuck utilizes the concept of thermal conduction from the wafer through a "high pressure", high conductivity gas (such as Helium, Argon, or some other inert gas) trapped between the wafer and the chuck. This has been shown to perform well, but requires a gas seal between the wafer and chuck.
The requirement for a seal leads to a different set of problems affecting temperature uniformity of the wafer. Seal material compatibility, limited pressure due to leaks, or non-uniform gas flow patterns all lead to control limitations and non-uniformities in the temperature profile of the wafer.
In order to provide a seal which holds the gas between the wafer and the chuck, an O-ring seal is used. The seal is generally slowly eaten up during processing, and particles from the seal contaminate the wafer. At high pressure, the material of the seal deteriorates and sticks to the wafer, which causes handling problems. In addition, the wafer bows upwards in a dome shape as the Helium pressure is increased. This causes total loss of contact except at the wafer clamping points and also causes variation in RF coupling between the wafer and the substrate electrode.